It is possible they received it from someone else (like from a Surprise Trade) and raised them. It is also possible they got a hacked one.
No excuse for if they turn out to have the same Tera type as the featured Tera Raid Battles though, like Ice for Inteleon or Bug for Samurott. Well, except that it may be a shared account, and different people are playing. This is a pretty common thing in the Splatoon games, for instance: pretty often, I'll find someone playing like they can barely hold a controller, and the next match, they're playing adeptly. It happens fairly often for multiple people of varying skill levels to use the same account.
I think it's more that they base Terastallization on how the in-story characters use them. Particularly with Gym Leaders on your first time through, their last Pokémon will primarily use moves of that Tera type. Katy's Teddiursa will only use Fury Cutter, Brassius's Sudowoodo will only use Trailblaze, Iono's Mismagius will only use Charge Beam, etc. This could give the impression that computer players are stupid and lock themselves into only moves of their Tera type.